By Nick Miles BBC correspondent in Mexico

Political leaders and academics begin a three day meeting in Rome on Monday to discuss issues surrounding global food security.

The United Nations World Food Summit will be looking at the spread of genetically modified crop strains as well as the immediate problem of famine in parts of southern Africa.

Our best guess is that the Mexican corn harvests have been contaminated by imported corn from the United States

Juan Martin Laboratory manager

Recent reports from Mexico suggest that despite a moratorium on planting GM corn strains, wild varieties have become contaminated by laboratory developed plants.

The most heavily contaminated area is in Oaxaca, where up to a quarter of corn samples have tested positive for GM.

GM contamination

The area is home to thousands of small subsistence farmers. High up in the central mountains of the state one such farmer, Olga Maldonado, is weeding her tiny plantation of corn. It's just forty metres square and barely provides enough food for her and her family.

"Corn is our way of life," she told me, as she worked under the harsh tropical sun. "Most of what we eat like tortillas and thick soups come from the crop."

Research has yet to prove GM foods are harmful

But six months ago Olga got a shock. Tests showed that her field had been contaminated with genetically modified corn. Now she says she's worried about the possible effects on the heath of her children.

"I'm concerned because our children eat corn all the time. We don't know whether it's safe to eat or not. The government has told us nothing about these GM plants. Absolutely nothing."

Just 16 kilometres away down a dirt track in the incongruous setting of rural Mexico, white coated workers at a hi-tech laboratory are testing samples of corn from across the country. The hum of electrical stirrers fills the air as the corn goes though the rigorous testing procedures.

This privately funded work has found widespread GM contamination in corn samples from across Mexico.

"Despite the moratorium on GM crop growing four years ago, many GM strains have slipped through the net," says Juan Martin, who's in charge at the lab.

We've had a number of different campaigns aimed at telling farmers not to use imported corn as seed

Victor Arambula, Mexican agriculture spokesman

"Our best guess is that the Mexican corn harvests have been contaminated by imported corn from the United States," he says.

"That corn was supposed to be for human consumption but farmers weren't aware of that and they sowed it in their fields."

Genie released

It's difficult to assess the Mexican Government's stance on the issue. State and federal governments seem to contradict each other on the significance and even the very existence of GM contamination.

Halberg fears food security will be affected

"If there is GM contamination then it's not because of any government failure," says Victor Arambula, spokesman for the Mexican agriculture ministry.

"We've had a number of different campaigns aimed at telling farmers not to use imported corn as seed.

"Over the last four years, we've made it perfectly clear to farmers that this contamination might happen."

However it happened, the GM genie is now out of the bottle. And many people are worried.

Whilst there is no universally accepted research proving any health risks from eating GM crops, environmentalists say that many GM strains encourage pests to become tolerant to insecticides.

"There will also be a drastic reduction in the variety of corn strains in Mexico," Boone Hallberg told me. Mr Hallberg, a sprightly 79-year-old US educated botanist, has been working with Mexican corn growers for almost four decades.

Maldonado's field has been contaminated

"Until now, when there's been a virus in other parts of the world, people have been able to come here to find strains resistant to the virus," he tells me as we walk through his organic corn fields.

"GM contamination will change all that. The thousands of varieties here will be lost forever, threatening food security around the world."

Meanwhile for small farmers like Olga the concerns are more immediate. Every mealtime is now a worrying time for her. She has to feed her children corn tortillas made from her now genetically modified crops by a farmer who unwittingly tainted her own land.

"I feel misled by the government," she says as she ladles some steaming corn drink into a bowl. "As usual in Mexico the needs of the farmers here have been overlooked".